1. Field of the Invention
The present invention relates in general to an apparatus for controlling a hybrid drive system of a motor vehicle, and more particularly to a technique for reducing an input torque of a transmission in such a hybrid drive system under a predetermined condition.
2. Discussion of the Related Art
There is known a hybrid drive system of a motor vehicle including (a) an engine operated by combustion of a fuel, (b) a motor/generator, (c) a transmission disposed between an assembly of the engine and the motor/generator and a drive wheel of the vehicle and having a variable speed ratio, and torque reduction control means for reducing the input torque of the transmission upon a shifting action of the transmission, under a predetermined condition. An example of such a hybrid drive system is disclosed in JP-A-6-319210, wherein the input torque of the transmission is temporarily reduced by controlling the torque of the motor/generator (electric motor), during a shifting action of the transmission under a predetermined condition, for instance, during an inertia phase of a shift-up action of the transmission while an accelerator pedal is in a depressed state.
However, the technique for reducing the input torque of the transmission by controlling the torque of the motor/generator is not necessarily satisfactory. For example, the reduction of the input torque is not possible when the amount of electric energy stored in an electric energy storage device for the motor/generator is not sufficient.